Recently, free rides on a wireless LAN (Local Area Network) have become a problem. The cases where such free rides occur are not limited to a case where connection authentication is not performed and a case where communication is not encrypted. For example, even when encryption is configured, if an encryption key is easily guessed (e.g., 12345678, etc.), unauthorized users can free ride easily. Further, even when a master device implements the “MAC address filtering” function that allows connections only to wireless terminals with allowed MAC (Media Access Control) addresses, a slave device that spoofs a MAC address may be allowed to free ride.
To address such a problem, Patent Literature 1 discloses a technique for preventing unauthorized access by checking a sequence number included in a frame or received radio wave strength at the time of communication with a wireless terminal.